Digital devices, such as digital valve positioners, are utilized to control electromechanical and hydraulic valve positioners to open, close and adjust the position of valves in a wide variety of industrial and commercial applications. Installed flow characteristics, such as the rate of fluid flow, through a valve mounted in a process pipeline are dependent upon a number of variables. These variables include the type of valve; the type of trim and the friction losses in the process equipment and piping system; the nature of the fluid, i.e. gaseous or liquid; and the temperature, pressure, density and viscosity of the fluid.
At present, digital valve positioners can provide data such as flow and differential pressure. However, the digital control devices of the prior art are not capable of collecting additional process data that is required to optimize the position of the valve, or the rate of change of the valve closure member, in order to maintain a steady-state throughout the process system.
Also missing from the prior art are auxiliary devices that include memory, or data storage devices, processors and transmitters, for processing this additional data in order to optimize a given valve's movement and final position in the context of a process loop and/or production facility.
For example, in a petroleum refinery or a petrochemical production facility, a number of different feedstreams, some of which may be liquid and some of which are gaseous, must be introduced into and removed from reactors, fractionators, distillation columns, catalytic crackers and the like, all at carefully controlled rates. If it is desired to increase or reduce the rate of production of a given end product, or of a by-product, changes to the valves controlling the flow-rates of the feedstreams, product take-off streams, heat exchange fluids and the like must be adjusted accurately under stable control.
In order to maximize controllability, certain operating companies assign significant pressure drops across control valves during project design, sometimes as much as 50% of the total system friction losses at design flow. The resultant changes from the inherent flow characteristic can be minimized by increasing the ratio of the control valve differential pressure at maximum versus minimum travel positions. However, this will result in significantly higher energy costs to run the plant.
Currently available digital control valve positioners offer a valve characterization feature to allow the installed flow characteristic to be modified manually.
As is well known to those skilled in the art, the adjustments necessary to bring such a system to a steady-state are numerous and time-consuming, can involve an element of danger, and can be costly in terms of both personnel time and loss in production efficiency. Moreover, the successful control of the process variables often depends upon the personal experience, care and talent of one or more engineers and plant workers. Following start-up of a new facility, or a new production system or loop, several months may be required for operations personnel to study each control loop and to optimize the operating data on a trial-and-error basis.
This task of properly matching the valve characteristic to the process requirement is an old problem that has required a time and labor-intensive effort and engineering knowledge to solve. The characterization of the valves required accurate process data collection, selection of the proper valve trim set on a trial-and-error basis and a careful analysis of the resultant installed flow characteristic based on measurements taken at the relevant on-site positions. Although optimum valve characterization is possible with conventional control systems on the market today, it is seldom actually achieved.
One proposal for controlling valves by seeking to produce linearity between the positioning signal controlling valve travel and the quantity of flow is set forth in U.S. Pat. No. 5,878,765 to Lange. This proposal entails obtaining a hydraulic profile of the flow based on measurements of the actual flow or the pressures upstream and downstream of the valve, and performs linearization using, for example, a specialized diaphragm or other mechanical element or specialized non-linear correction circuitry.
The present invention uses commercially available technology to go beyond these prior methods to a method and apparatus that automatically linearizes the non-linear relation between the input control signal of the digital positioner and the flow over the required travel range. The present invention is also adapted to automatically obtain constant static loop gain for, e.g., two or more valves installed in parallel.